How to select all strips in a channel via python?
Set strip.select to True if strip.channel is "active channel".

Test Script with  active_channel = 1. Note I've simply set the active strip to None if it is not in the desired "active channel".  Change this to suit.

import bpy

context = bpy.context
scene = context.scene

active_channel = 1

sed = scene.sequence_editor
# if active strip isn't in active_channel set to None.
if getattr(sed.active_strip, "channel", -1) != active_channel:
    sed.active_strip = None

sequences = sed.sequences_all
# select all strips in active channel
for strip in sequences:
    strip.select = strip.channel == active_channel